The antimicrobial properties of a number of inorganic materials, especially metals such as silver, copper, zinc, mercury, tin, gold, lead, bismuth, cadmium, chromium and thallium, have long been known. Certain of these metals, especially silver, zinc, gold and copper, have enjoyed greater success due to their relatively low environmental and toxicological effects and high antimicrobial activity. More recently, antimicrobial agents which incorporate ionic forms of these metals, especially through an ion-exchange type mechanism, have achieved greater attention due to the higher bioactivity of the ionic versus the metallic form of these metals in an antimicrobial application. Exemplary ion-exchange type antimicrobial agents include those wherein the ion-exchange carrier particles are ceramic particles including zeolites, hydroxy apatites, zirconium phosphates and the like. Antimicrobial agents based on zeolite carriers are disclosed in, for example, U.S. Pat. Nos. 4,911,898; 4,911,899; 4,938,955; 4,906,464; and 4,775,585. Antimicrobial zirconium phosphates include those disclosed in, for example, U.S. Pat. Nos. 4,025,608 and 4,059,679 and the Journal of Antibacterial Antifungal Agents Vol. 22, No. 10, pp. 595-601, 1994. Finally, antimicrobial hydroxyapatites powders include those disclosed in U.S. Pat. Nos. 5,009,898 and 5,268,174, among others.
Despite the relative, though restrained, success of these antimicrobial agents as additives for polymer resins and the several patent publications teaching or suggesting their utility in coatings, these antimicrobial agents have actually found limited commercial utility in coating applications. Though, as noted below, the problems associated with their use are common to both coating and molding/compounding applications, the impact is far greater on coating applications; especially since coating applications are more visible and, indeed, are intended to provide a decorative or esthetic appearance to the substrate being coated.
The first significant problem associated with ion-exchange type antimicrobial agents is the fact that, in non-hydrophilic compositions, only that amount of the antimicrobial agent which is present at or proximate to, on a molecular scale, the surface of the polymer substrate or coating actually participates in providing antimicrobial activity. This inactivity arises from the fact that these antimicrobial agents rely upon an ion-exchange mechanism that is facilitated by, if not reliant upon, the presence of water or a similar medium, especially an aqueous based or containing medium, which transports the exchange cations in and the antimicrobial metal cations out of the ceramic particles. Non-hydrophilic polymers do not posses the internal moisture or water needed for this transport and, consequently, those antimicrobial particles that lie below the surface are inactive: at least until they become exposed due to wear and/or erosion.
Perhaps the more significant problem associated with ion-exchange type antimicrobial agents is their tendency to cause discoloration of the polymer resin or coating composition into which they are incorporated. In polymer compounding applications discoloration is often immediate; arising from an interaction of silver, especially silver ions on the surface of the particles and/or exchanging out of the particles (due to the presence of moisture in the compound pre-mix or the air), with other compounds, ions, and the like present in the polymer and/or the compound pre-mix into which the antimicrobial agent is being incorporated, particularly when conducted in the presence of moisture, including conditions of high humidity. Whether immediate or not, whether in a compounded polymer or polymer coating, discoloration also manifests itself over time as a result of the occurrence of the aforesaid interaction between the silver and other reactive components during compounding or cure, as appropriate, and/or as a result of various environmental conditions, especially high humidity and/or UV light, acting upon the so-formed silver compounds. While the use of these antimicrobial agents in polymer resins can be directed to articles or components that are not readily visible, coatings by design are intended for visual impact. Thus, discoloration is of utmost concern with coatings.
Numerous efforts have been undertaken to improve the use of ion-exchange type antimicrobial agents in coatings and polymer compositions. So far, such efforts have found limited success: oftentimes achieving success relative to one issue, but not both issues. For example, in U.S. Pat. No. 6,436,422, Trogolo et. al., employed hydrophilic polymer coatings so as to ensure that all of the antimicrobial agent within the coating was active and participating in providing antimicrobial efficacy. However, hydrophilic polymers, whether as a coating or molded material, have limited use due to their severely limited physical and performance properties. Furthermore, discoloration persisted.
Alternatively, where efforts have been made to address both issues, they were addressed only to a limited extent. For example, subsequent developments by Trogolo et. al., (see e.g., US 2003-0118664) achieved some, though limited success against both issues. Specifically, Trogolo et. al. found that by encapsulating the antimicrobial particles in a hydrophilic polymer they were able to increase the effective particle size of the antimicrobial agent, thereby increasing the likelihood that any given antimicrobial particle would be present at or proximate to the surface of the polymer substrate or coating into which it is incorporated. These antimicrobial additives also had the benefit of slowing down, if not reducing, discoloration since the hydrophilic polymer encapsulating material essentially isolated the antimicrobial active from the polymer resin or coating materials into which it was being incorporated during the critical compounding process. Furthermore, what little, if any, interaction or reaction that did take place was essentially limited to taking place in the encapsulating material and did not transition into the polymer matrix itself. Though the encapsulated additives of Trogolo et. al. provide many benefits, there are still lingering concerns with, among other issues, the introduction of yet another ingredient, the hydrophilic polymer, into the matrix polymer composition to be rendered antimicrobial; the affinity or incompatibility of the hydrophilic polymer with the matrix polymer; inactive particles in thick molded parts or coating applications; and long-term discoloration arising from the subsequent release of the antimicrobial ions from the antimicrobial additive particles themselves into or at the interface with the matrix polymer.
Generally speaking, it can be seen that antimicrobial coating systems require a compromise amongst several desirable properties. Hydrophilic polymer coatings are especially suited for use with ion-exchange type antimicrobial agents; however, these coatings are typically easily abraded in any erosive environment and do not give lasting protection. Non-hydrophilic polymers provide improved wear resistance due to their stronger physical performance characteristics and properties but suffer, comparatively, in terms of their antimicrobial activity. These problems can be exacerbated by surfactants and leveling agents commonly used in coating systems and designed to form a skin at the surface of the coating to control surface finish. This same skin can also form over the antimicrobial agent; thereby potentially inactivating the same. Generally this skinning effect results from two circumstances. The first is where the particle is suspended in the coating and the coating forms a film over the surface of the particle even though the particle is at or proximate to the surface of the top coat. The second is where the coating is such that even where the antimicrobial particles protrude above the surface of the coating matrix (as for example where the particles have a greater diameter than the thickness of the coating), the surface tension forces of the coating are such that it still forms a film over the particles. Although greater amounts of the antimicrobial agent will provide a higher concentration of the antimicrobial agent at the surface; this is more costly and increases the likelihood of discoloration since the polymer system has even more silver content.
Because silver based antimicrobial coatings, especially those involving silver salts and/or ionic silver, are especially prone to discoloration, especially over time, their utility is severely limited to those applications which have short life expectancies and/or are not readily visible or, if visible, are not employed where color stability and esthetics are important.
Thus, there remains a need to provide a silver based antimicrobial coating in a form that is suitable to impart antimicrobial properties without the accompanying problems of the prior art. More specifically, there remains a need to provide a silver based antimicrobial coating that provides good long-term antimicrobial activity and color stability.